Baby teeth hold clues to the harms of toxic metals for infants — and older kids

kuppa_rock/Getty Images Baby teeth have a value that goes beyond money from the tooth fairy. Scientists say they also hold the key to figuring out what metals in the environment infants were exposed to — and how they affect not just developing baby brains, but behavior in adolescents. "It seems like an impossible question to answer," says Dr. Manish Arora, a professor of environmental medicine at the Icahn School of Medicine at Mount Sinai. "Luckily for us, baby teeth are this amazing organ." He says baby teeth work kind of like tree growth rings. They start to form in utero, beginning in the second trimester. And as they develop, layer by layer, they incorporate trace amounts of metals they're exposed to — in the womb and early life. That's when the brain is most vulnerable to toxic metals. In a new study in the journal Science Advances, Arora and his colleagues used lasers to decode those layers in the baby teeth shed by 500 children in Mexico City. That allowed them to create a timeline of what neurotoxic metals those children were exposed to, week by week, even before they were born. Connecting teeth to the brain The children in the study have been followed since their mothers were pregnant. As the children reached adolescence, the researchers also took detailed behavior assessments for some of the kids and MRI scans of their brains. While the baby teeth allowed the researchers to figure out the timeline of exposures, the brain scans helped them link those exposures to impacts on brain development, says study co-author Megan Horton, also a professor of environmental medicine at Mount Sinai. Want the latest stories on the science of healthy living? Subscribe to NPR's Health newsletter. "Using MRI allows us to look at what those exposures are doing to the brain in terms of its structure, connections and how different areas of the brain communicate," Horton says. The researchers looked at exposures to nine metals common in the environment, including lead, zinc and copper. Some, like manganese, are essential to growing bodies in trace amounts, but too much can be harmful to developing brains. Previous studies have linked exposure to neurotoxic metals in early life to changes in the structure of the brain and increased risk of mental health problems later in life. When the brain is most vulnerable What they discovered is that it's not just how much of these metals babies are exposed to that matters, but when that exposure happens. "We found that exposure to this metal mixture during this critical period of around 6 to 9 months of development was strongly associated with negative changes in behavior in these adolescents," including inattention and hyperactivity, Horton says. They also found a strong link to changes in the brain, including a decrease in overall brain volume and changes in the way different areas of the brain connect with each other. They also found abnormalities in the brain's white matter, which is important to the speed and efficiency of thought. Horton says 6 to 9 months of age is a period of rapid growth and change for infant brains. "Connections between brain cells are being rapidly formed and refined. Different brain regions are starting to communicate more efficiently," she says. At the same time, babies are also starting to crawl and transitioning from breastmilk or formula to solid foods. "So there's a lot happening in the brain and there's also a lot happening in the child's environment that might make this a very vulnerable and unique period of development," Horton says. The researchers say many of the metals in the study are commonly found in the foods we eat, the water we drink, and our built environment. They say the findings show that it's not just the amount of these metals that children are exposed to, but also the cumulative effect of these exposures, that presents a risk to developing brains. And the timing of that exposure is critical. "It's not just how much you were expose